1. Field of the Invention
This invention relates to scale inhibition in non-carbonate reservoirs.
Relatively low concentrations of water-soluble, organic scale inhibitors are known to reduce the rate of scale formation in and around the bottom of a producing well. Scales are slightly soluble inorganic salts, such as calcium or barium sulfates or calcium carbonate, etc. The mechanism of the inhibition is probably a combination of sequestering or complexing of multivalent cations and an absorption of inhibitor onto solid surfaces such as the rapid growth planes of newly formed crystallites. Although a wide variety of materials inhibit crystal growth, the compounds most commonly used in well treatments are organic phosphates or phosphonates, adducts of acrylic acid or the like. Where supersaturated, or scale-forming waters are encountered in an oil production operation, scale inhibitors are commonly injected or "squeezed" into the reservoir formation. The squeezing amounts to injecting the inhibitor and, usually over-flushing the treated zone with water. The well is placed back on production and the entrainment of the inhibitor in the produced water protects the wellbore and downhole equipment from scale build-up.
In recent years a procedure called "precipitation squeeze" has been developed. In this process, the scale inhibitor reacts or is reacted to form an insoluble salt which precipitates in the pores of the formation rock. For example, phosphonates and polymeric carboxylates can be precipitated as the calcium salt. Other inhibitors such as phosphate esters are not suitable since they do not readily form insoluble calcium salts. In practice, an acidic solution of a phosphonate, for example, is squeezed down a wellbore into a limestone or dolomite-containing reservoir. Here the acid acts on the carbonate to both neutralize the acid and put calcium ions into solution: EQU 2H(Phos)+CaCO.sub.3 Ca(Phos)+H.sub.2 CO.sub.3
With rising pH and increasing calcium ion concentration, precipitation of calcium phosphonate takes place within the pore spaces of the rock. Following precipitation, the well is returned to production normally after 24 hours.
Precipitation squeeze treatments have proven very successful in carbonate reservoirs, generally reaching a longer useful life than with conventional adsorption-type squeezes. However, this technique is not applicable in non-carbonate reservoirs where only a limited amount of calcium is available downhole. The use of precipitation squeezes in non-carbonate (sand) reservoirs would be expected to provide the same extended life treatment now seen in carbonate reservoirs. To achieve such a squeeze in sandstones, both calcium ion and phosphonate must be included in the solution placed in the reservoir.
2. The Prior Art
U.S. Pat. No. 3,483,925 discloses a process in which a viscosity-increasing polymer (having a molecular weight between about 100,000 and about 10,000,000) is injected into a formation followed by the injection of an organic phosphonate scale inhibitor. Examples of polymers used are hydrolyzed acrylamide and copolymers of acrylamide.
U.S. Pat. No. 3,167,123 discloses injecting a high viscosity fluid, such as viscous crude oil into a formation followed by injection of a low viscosity corrosion inhibitor.
U.S. Pat. No. 4,860,829 discloses injecting a phosphonate inhibitor and a metal chelate into a non-carbonate reservoir to inhibit scaling.